Does this double your reception signal? What would you use on the antenna mast to join the coax cables....a splitter? I'd need something with 2 inputs? I was thinking if it works to add that free stealth antenna up there.
In certain situations, yes it can help. Stacking two antennas together is recommended when trying to reduce ghosting/multipath. Hooking two different antennas up is not recommended. Normally you would do it with two exactly identical antennas pointing exactly the same direction and spaced a very specific distance apart. Done correctly you can achieve a theoretical 3db increase in gain. Done incorrectly, it can (and probably will) actually hurt your reception.
As part of my Voom install, they installed two antennas. The reason why is the transmitter stations are 67 degrees and 212 degrees. They are both way different types of antennas. The first is a large directional antenna, the second is a Yagi antenna. They're stacked. Becasue of this install, I am able to get three HD digital stations (ABC, NBC, and CBS). Voom did not charge me for these antennas. The antennas are diplexed into the satellite cable. I believe if the diplexor was taken off I would receive the fourth station (PBS). So, yes, two antennas spliced together will work. And you may even get Voom to do it for you.